Poly-4-hydroxybutyrate (P4HB) and copolymers thereof can be produced using transgenic fermentation methods, see, for example, U.S. Pat. No. 6,548,569 to Williams et al., and are produced commercially, for example, 20 by Tepha, Inc. (Lexington, Mass.). Poly-4-hydroxybutyrate (P4HB, TephaFLEX® biomaterial) is a strong, pliable thermoplastic polyester that, despite its biosynthetic route, has a relatively simple structure. Upon implantation, P4HB hydrolyzes to its monomer, and the monomer is metabolized via the Krebs cycle to carbon dioxide and water.
The polymer belongs to a larger class of materials called polyhydroxyalkanoates (PHAs) that are produced by numerous microorganisms (see, for example, Steinbüchel A., et al. Diversity of Bacterial Polyhydroxyalkanoic Acids, FEMS Microbial. Lett. 128:219-228 (1995)). In nature these polyesters are produced as storage granules inside cells, and serve to regulate energy metabolism. They are also of commercial interest because of their thermoplastic properties, and relative ease of production using recombinant bacteria or plants. Chemical synthesis of P4HB has been attempted, but it has been impossible to produce the polymer with a sufficiently high molecular weight that is necessary for most applications (see Hori, Y., et al., Polymer 36:4703-4705 (1995) and Houk, K. N., et al., J. Org. Chem., 2008, 73 (7), 2674-2678).
U.S. Pat. Nos. 6,245,537, 6,623,748, 7,244,442, and 8,231,889 describe methods of making PHAs with little to no endotoxin, which are suitable for medical applications. U.S. Pat. Nos. 6,548,569, 6,838,493, 6,867,247, 7,268,205, 7,179,883, 7,268,205, 7,553,923, 7,618,448 and 7,641,825 and WO 2012/064526 describe use of PHAs to make medical devices. Copolymers of P4HB include 4-hydroxybutyrate copolymerized with 3-hydroxybutyrate or glycolic acid (U.S. Pat. No. 8,039,237 to Martin and Skraly, U.S. Pat. No. 6,316,262 to Huisman et al., and U.S. Pat. No. 6,323,010 to Skraly et al.). Methods to control molecular weight of PHA polymers have been disclosed by U.S. Pat. No. 5,811,272 to Snell et al.
PHAs with controlled degradation and degradation in vivo of less than one year are disclosed by U.S. Pat. Nos. 6,548,569, 6,610,764, 6,828,357, 6,867,248, and 6,878,758 to Williams et al. and WO 99/32536 to Martin et al. Applications of P4HB have been reviewed in Williams, S. F., et al., Polyesters, III, 4:91-127 (2002), and by Martin, D. et al. Medical Applications of Poly-4-20 hydroxybutyrate: A Strong Flexible Absorbable Biomaterial, Biochem. Eng. J. 16:97-105 (2003). Medical devices and applications of P4HB have also been disclosed by WO 00/56376 to Williams et al. Several patents including U.S. Pat. Nos. 6,555,123, 6,585,994, and 7,025,980 describe the use of PHAs in tissue repair and engineering.
U.S. Pat. No. 8,034,270 to Martin et al. discloses monofilament and multifilament knitted meshes of P4HB produced by knitting monofilament and multifilament fibers of P4HB. WO 2011/119742 to Martin et al. discloses P4HB monofilament and multifilament fiber, coatings and spin finishes for these fibers, and medical devices made from P4HB monofilament and multifilament fibers. U.S. Pat. No. 8,016,883 to Coleman et al. discloses methods and devices for rotator cuff repair, including medical devices containing knitted meshes of P4HB and non-wovens made from P4HB multifilament fibers.
U.S. Pat. No. 8,287,909 to Martin et al. discloses medical devices containing melt-blown non-wovens of poly-4-hydroxybutyrate and copolymers thereof with average fiber diameters of 1 μm to 50 μm. Notably, the process of melt blowing results in a significant drop in the molecular weight of P4HB which can be a disadvantage if it is desirable to retain mechanical properties, such as burst strength, in vivo, for a prolonged period of time.
WO 2011/159784 to Cahil et al. discloses medical devices containing dry spun non-wovens of P4HB and copolymers thereof, and continuous processing methods for their preparation. The fibers of the non-wovens have average diameters in the micron range.
A low melting and high modulus electrospun scaffold made from a blend of P4HB and poly(ε-caprolactone) (80:20), and spun from a 7.5 wt./v-% THF solution, is disclosed by Vaz, C. M. et al., Novel Electrospun P4HB:PCL Scaffold for Aortic Valve Tissue Engineering, Poster Presentation (2004), Eindhoven University of Technology. Notably, blending poly(ε-caprolactone) with P4HB significantly depresses the melting temperatures of both poly(ε-caprolactone) and P4HB. The blended scaffold has a melting point of 40° C., which is significantly lower than the melting point of 58-60° C. for P4HB, and as such has very limited commercial use due to its thermal instability at relatively low temperatures. Since both polymers are thermoplastics, significant physical changes occur upon implantation of this blend in the body, since the melting temperature of the blend is only 3° C. above body temperature, and the blend would likely melt in a patient with an elevated temperature.
WO 95/23249 to Noda et al. discloses fabrics prepared from other polyhydroxyalkanoates, namely, poly-3-hydroxybutyrate (PHB) and poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) by dry spinning for use in non-medical applications such as disposable absorbent articles, including diapers, incontinence articles, and sanitary napkins. These materials, however, have substantially different thermal and physical properties than poly-4-hydroxybutyrate and copolymers thereof. For example, PHB has a melting point and glass transition temperature of approx. 180° C. and 1° C., respectively, and an elongation to break of about 3%, whereas P4HB has a melting point of 60° C., a glass transition temperature of approx. −51° C., and elongation to break of around 1,000%. As such, PHB is a brittle polymer that has properties resembling polystyrene whereas P4HB is a strong but extensible polymer similar to low density polypropylene. Furthermore, PHB and PHBV have also been reported to degrade very slowly in vivo with material still present after 24 months (Duvernoy, O. et al. A biodegradable patch used as a pericardial substitute after cardiac surgery: 6- and 24-month evaluation with CT, Thorac. Cardiovacs. Surgeon, 43:271-274 (1995)), and are therefore not well suited for many in vivo surgical applications.
In the practice of surgery there currently exists a need for structures containing ultrafine degradable fibers with improved performance. These structures can be used, for example, in both soft and hard tissue repair, to reinforce tissue structures, to separate tissues, to regenerate tissues, and to replace tissues. The ultrafine fibers can also be used as components of other device structures.
Ultrafine fibers may be made by electrospinning. This process produces unusually small diameter fibers with average diameters in the nanometer range typically from about 10 nm to 10 μm or more narrowly from about 50 nm to about 5 μm. In contrast, melt extrusion and dry spinning typically produce fibers with diameters in the micron range.
The equipment for electrospinning typically comprises a spinneret connected to a high voltage direct current, and a grounded collector plate. In the laboratory, a hypodermic syringe needle is frequently used as the spinneret, but a plurality of nozzles may also be used. To make ultrafine fibers, the polymer material to be electrospun is dissolved in a solvent, and pumped through the spinneret under a constant pressure or flow rate. As the polymer material solution emerges from the spinneret, the liquid surface of the solution is charged as it becomes subjected to the electric field. As the strength of the field increases and overcomes the surface tension, the hemispherical surface of the polymer material solution at the spinneret begins to elongate, and forms a conical shape known as the Taylor cone. At a critical point, when the electric field force overcomes the surface tension at the spinneret, a charged jet of the polymer material solution is ejected from the Taylor cone and travels toward the collector plate, provided the molecular cohesion of the solution is sufficiently strong. During transit, the solvent evaporates from the jet to form a charged ultrafine fiber that is deposited at the collector plate. If desired, compressed air or other gas may be injected adjacent to the spinneret to help form the desired nanofibers.
The properties of electrospun fibers may be tailored by varying certain parameters of the electrospinning process. These include (i) solvent and solution properties, such as viscosity, conductivity, evaporation rate, concentration and surface tension, (ii) equipment set up conditions, such as the distance between the spinneret and collector plate, the electric potential applied, and the pressure or flow rate of the polymer material solution, (iii) atmospheric conditions, such as temperature, humidity, and any applied air velocity between the spinneret and collector plate, and (iv) polymer properties such as Mw, composition, melting properties, thermal properties and crystallization rate. It is also possible to set up the electrospinning equipment such that the spinneret and/or the collector plate are moving relative to one another in order to produce different electrospun structures and shapes.
A number of absorbable materials have been used to produce ultrafine fibers for use in surgery. For example, ultrafine fibers have been made from polyglycolic acid (PGA) or copolymers containing lactic acid. These materials do not, however, have ideal properties for many procedures and applications. For example, ultrafine fibers made from glycolic acid containing polymers degrade quickly, are moisture sensitive and release acidic degradation products that can cause inflammatory reactions.
It is therefore an object of the invention to provide continuous processes for production of ultrafine fibers of P4HB and copolymers thereof, which can be incorporated into or formed into medical devices with excellent physical and mechanical properties for medical applications.
It is an object of the present invention to provide methods to produce electrospun ultrafine fibers of absorbable P4HB and copolymers thereof that have average diameters from 10 nm to 10 μm and more preferably from 50 nm to 5 μm, and high surface areas to volume ratios.
It is a further object of the present invention to provide methods to produce electrospun ultrafine fibers of absorbable P4HB and copolymers thereof without substantial loss of the polymer molecular weight during processing.
It is still a further object of the present invention to provide methods to produce electrospun ultrafine fibers of absorbable P4HB and copolymers thereof with a high degree of molecular orientation in the fibers.
It is yet another object of the present invention to provide methods to produce electrospun ultrafine fibers of absorbable P4HB wherein the polymer has a melting point which is higher than the melting point of P4HB fibers formed by dry spinning.
It is another object of the present invention to provide continuous processes to produce medical devices containing ultrafine fibers of P4HB and copolymers thereof by electrospinning, including processes to form medical devices by coating other materials and scaffolds with electrospun ultrafine fibers, and processes to electrospin P4HB and copolymers thereof into ultrafine fibers without substantial loss of molecular weight during the spinning process.